1. Field of the Invention
The present invention generally relates to a sampled video signal generating device, and more particularly to a sampled video signal generating device in which deviation which may be caused among sampled video signals due to differences in circuit characteristics among channels is reduced. The present invention has particular applicability to a sampled video signal generating device for a Liquid Crystal Display.
2. Description of the Background Art
In a high quality television system known as a television system for the next generation, the so-called "High Vision", resolution is increased as compared to present television systems, resulting in providing vivid image to viewers. That is to say, in High Vision, the number of horizontal scanning lines is defined to be 1125 or more for one frame, interlace is set at 2:1 or no-interlace, and aspect ratio at 16:9. On the other hand, in the NTSC system which is the current television system, the number of horizontal scanning lines is determined to be 525 for one frame, and interlace ratio is set at 2:1, and aspect ratio at 4:3. As a result, the amount of information handled in the High Vision system is about 5 times or more than that in the NTSC system.
As described above, since it is necessary to handle a larger amount of information in the high quality television system, various devices have been invented for video signal processing. For example, in a video signal processing circuit for supplying video signals to a liquid crystal display according to the high quality television system, the number of picture elements is considerably increased, so that the video signals should be sampled at an extremely high speed. Video signals sampled at high speed are provided as picture element signals to the horizontal scanning circuits of the respective liquid crystal panels in the liquid crystal display. As the digital horizontal scanning LSI circuit, one of a sample-and-hold type is employed because of its excellent technical property.
However, as a digital horizontal LSI has a limit in speed of high vision signal scanning, a plurality simultaneous scanning system which divides a single video signal into a plurality of channels is employed. That is to say, a continuous video signal is sampled at predetermined time intervals to produce sampled video signals divided into a plurality of channels. The produced signals are simultaneously inputted to the horizontal scanning circuits. In other words, the operation speed of the horizontal scanning circuit connected to each channel can be reduced to 1/n by dividing a single video signal into sampled video signals of n channels. Since the operation speed of the horizontal scanning circuit connected to each channel is reduced, desired horizontal resolution can be easily implemented without reducing video frequency bandwidth by the horizontal scanning circuit. Here, the phase disorder among the n channels should be restrained within a permitted value corresponding to the sampling frequency.
As described above, since a single video signal is divided into a plurality of channels by sampling and the video signal processing for obtaining sampled video signals is performed for each channel, differences in circuit characteristics among signal processing circuits for handling video signals of the respective channels is not acceptable. That is to say, when deviation of circuit characteristics exists among signal processing circuits, the picture quality of an image obtained according to video signals generated from the respective signal processing circuits is degraded. Such deviation of circuit characteristics occurs not only because of various differences of manufacturing conditions but also can be produced due to environmental conditions in use such as temperature and moisture.